Coupler-Assisted Leakage Reduction for Scalable Quantum Error Correction with Superconducting Qubits

  1. Xiaohan Yang,
  2. Ji Chu,
  3. Zechen Guo,
  4. Wenhui Huang,
  5. Yongqi Liang,
  6. Jiawei Liu,
  7. Jiawei Qiu,
  8. Xuandong Sun,
  9. Ziyu Tao,
  10. Jiawei Zhang,
  11. Jiajian Zhang,
  12. Libo Zhang,
  13. Yuxuan Zhou,
  14. Weijie Guo,
  15. Ling Hu,
  16. Ji Jiang,
  17. Yang Liu,
  18. Xiayu Linpeng,
  19. Tingyong Chen,
  20. Yuanzhen Chen,
  21. Jingjing Niu,
  22. Song Liu,
  23. Youpeng Zhong,
  24. and Dapeng Yu
Superconducting qubits are a promising platform for building fault-tolerant quantum computers, with recent achievement showing the suppression of logical error with increasing code
size. However, leakage into non-computational states, a common issue in practical quantum systems including superconducting circuits, introduces correlated errors that undermine QEC scalability. Here, we propose and demonstrate a leakage reduction scheme utilizing tunable couplers, a widely adopted ingredient in large-scale superconducting quantum processors. Leveraging the strong frequency tunability of the couplers and stray interaction between the couplers and readout resonators, we eliminate state leakage on the couplers, thus suppressing space-correlated errors caused by population propagation among the couplers. Assisted by the couplers, we further reduce leakage to higher qubit levels with high efficiency (98.1%) and low error rate on the computational subspace (0.58%), suppressing time-correlated errors during QEC cycles. The performance of our scheme demonstrates its potential as an indispensable building block for scalable QEC with superconducting qubits.

Deterministic quantum teleportation between distant superconducting chips

  1. Jiawei Qiu,
  2. Yang Liu,
  3. Jingjing Niu,
  4. Ling Hu,
  5. Yukai Wu,
  6. Libo Zhang,
  7. Wenhui Huang,
  8. Yuanzhen Chen,
  9. Jian Li,
  10. Song Liu,
  11. Youpeng Zhong,
  12. Luming Duan,
  13. and Dapeng Yu
Quantum teleportation~cite{Bennett1993} is of both fundamental interest and great practical importance in quantum information science. To date, quantum teleportation has been implemented
in various physical systems~\cite{Pirandola2015}, among which superconducting qubits are of particular practical significance as they emerge as a leading system to realize large-scale quantum computation~\cite{Arute2019,Wu2021}. Nevertheless, the number of superconducting qubits on the same chip is severely limited by the available chip size, the cooling power, and the wiring complexity. Realization of quantum teleportation and remote computation over qubits on distant superconducting chips is a key quantum communication technology to scaling up the system through a distributed quantum computational network~\cite{Gottesman1999,Eisert2000,Jiang2007,Kimble2008,Monroe2014}. However, this goal has not been realized yet in experiments due to the technical challenge of making a quantum interconnect between distant superconducting chips and the inefficient transfer of flying microwave photons over the lossy interconnects~\cite{Kurpiers2018,Axline2018,Campagne2018,Magnard2020}. Here we demonstrate deterministic teleportation of quantum states and entangling gates between distant superconducting chips connected by a 64-meter-long cable bus featuring an ultralow loss of 0.32~dB/km at cryogenic temperatures, where high fidelity remote entanglement is generated via flying microwave photons utilizing time-reversal-symmetry~\cite{Cirac1997,Korotkov2011}. Apart from the fundamental interest of teleporting macroscopic superconducting qubits over a long distance, our work lays a foundation to realization of large-scale superconducting quantum computation through a distributed computational network~\cite{Gottesman1999,Eisert2000,Jiang2007,Kimble2008,Monroe2014}.

Low-loss interconnects for modular superconducting quantum processors

  1. Jingjing Niu,
  2. Libo Zhang,
  3. Yang Liu,
  4. Jiawei Qiu,
  5. Wenhui Huang,
  6. Jiaxiang Huang,
  7. Hao Jia,
  8. Jiawei Liu,
  9. Ziyu Tao,
  10. Weiwei Wei,
  11. Yuxuan Zhou,
  12. Wanjing Zou,
  13. Yuanzhen Chen,
  14. Xiaowei Deng,
  15. Xiuhao Deng,
  16. Changkang Hu,
  17. Ling Hu,
  18. Jian Li,
  19. Dian Tan,
  20. Yuan Xu,
  21. Fei Yan,
  22. Tongxing Yan,
  23. Song Liu,
  24. Youpeng Zhong,
  25. Andrew N. Cleland,
  26. and Dapeng Yu
Scaling is now a key challenge in superconducting quantum computing. One solution is to build modular systems in which smaller-scale quantum modules are individually constructed and
calibrated, and then assembled into a larger architecture. This, however, requires the development of suitable interconnects. Here, we report low-loss interconnects based on pure aluminium coaxial cables and on-chip impedance transformers featuring quality factors up to 8.1×105, which is comparable to the performance of our transmon qubits fabricated on single-crystal sapphire substrate. We use these interconnects to link five quantum modules with inter-module quantum state transfer and Bell state fidelities up to 99\%. To benchmark the overall performance of the processor, we create maximally-entangled, multi-qubit Greenberger-Horne-Zeilinger (GHZ) states. The generated inter-module four-qubit GHZ state exhibits 92.0\% fidelity. We also entangle up to 12 qubits in a GHZ state with 55.8±1.8% fidelity, which is above the genuine multipartite entanglement threshold of 1/2. These results represent a viable modular approach for large-scale superconducting quantum processors.

Conditional coherent control with superconducting artificial atoms

  1. Chang-Kang Hu,
  2. Jiahao Yuan,
  3. Bruno A. Veloso,
  4. Jiawei Qiu,
  5. Yuxuan Zhou,
  6. Libo Zhang,
  7. Ji Chu,
  8. Orkesh Nurbolat,
  9. Ling Hu,
  10. Jian Li,
  11. Yuan Xu,
  12. Youpeng Zhong,
  13. Song Liu,
  14. Fei Yan,
  15. Dian Tan,
  16. R. Bachelard,
  17. Alan C. Santos,
  18. C. J. Villas-Boas,
  19. and Dapeng Yu
Controlling the flow of quantum information is a fundamental task for quantum computers, which is unpractical to realize on classical devices. Coherent devices which can process quantum
states are thus required to route the quantum states yielding the information. In this paper we demonstrate experimentally the smallest quantum transistor for superconducting processors, composed of collector and emitter qubits, and the coupler. The interaction strength between the collector and emitter is controlled by tuning the frequency and the state of the gate qubit, effectively implementing a quantum switch. From the truth-table measurement (open-gate fidelity 93.38%, closed-gate fidelity 98.77%), we verify the high performance of the quantum transistor. We also show that taking into account the third energy level of the qubits is critical to achieving a high-fidelity transistor. The presented device has a strong potential for quantum information processes in superconducting platforms.

Entanglement purification and protection in a superconducting quantum network

  1. Haoxiong Yan,
  2. Youpeng Zhong,
  3. Hung-Shen Chang,
  4. Audrey Bienfait,
  5. Ming-Han Chou,
  6. Christopher R. Conner,
  7. Étienne Dumur,
  8. Joel Grebel,
  9. Rhys G. Povey,
  10. and Andrew N. Cleland
High-fidelity quantum entanglement is a key resource for quantum communication and distributed quantum computing, enabling quantum state teleportation, dense coding, and quantum encryption.
Any sources of decoherence in the communication channel however degrade entanglement fidelity, thereby increasing the error rates of entangled state protocols. Entanglement purification provides a method to alleviate these non-idealities, by distilling impure states into higher-fidelity entangled states. Here we demonstrate the entanglement purification of Bell pairs shared between two remote superconducting quantum nodes connected by a moderately lossy, 1-meter long superconducting communication cable. We use a purification process to correct the dominant amplitude damping errors caused by transmission through the cable, with fractional increases in fidelity as large as 25%, achieved for higher damping errors. The best final fidelity the purification achieves is 94.09±0.98%. In addition, we use both dynamical decoupling and Rabi driving to protect the entangled states from local noise, increasing the effective qubit dephasing time by a factor of 4, from 3 μs to 12 μs. These methods demonstrate the potential for the generation and preservation of very high-fidelity entanglement in a superconducting quantum communication network.

Scalable method for eliminating residual ZZ interaction between superconducting qubits

  1. Zhongchu Ni,
  2. Sai Li,
  3. Libo Zhang,
  4. Ji Chu,
  5. Jingjing Niu,
  6. Tongxing Yan,
  7. Xiuhao Deng,
  8. Ling Hu,
  9. Jian Li,
  10. Youpeng Zhong,
  11. Song Liu,
  12. Fei Yan,
  13. Yuan Xu,
  14. and Dapeng Yu
Unwanted ZZ interaction is a quantum-mechanical crosstalk phenomenon which correlates qubit dynamics and is ubiquitous in superconducting qubit system. It adversely affects the quality
of quantum operations and can be detrimental in scalable quantum information processing. Here we propose and experimentally demonstrate a practically extensible approach for complete cancellation of residual ZZ interaction between fixed-frequency transmon qubits, which are known for long coherence and simple control. We apply to the intermediate coupler that connects the qubits a weak microwave drive at a properly chosen frequency in order to noninvasively induce ac Stark shift for ZZ cancellation. We verify the cancellation performance by measuring vanishing two-qubit entangling phases and ZZ correlations. In addition, we implement randomized benchmarking experiment to extract the idling gate fidelity which shows good agreement with the coherence limit, demonstrating the effectiveness of ZZ cancellation. Our method allows independent addressability of each qubit-qubit connection, and is applicable to both non-tunable and tunable coupler, promising better compatibility with future large-scale quantum processors.

Optimal charging of a superconducting quantum battery

  1. Chang-Kang Hu,
  2. Jiawei Qiu,
  3. Paulo J. P. Souza,
  4. Jiahao Yuan,
  5. Yuxuan Zhou,
  6. Libo Zhang,
  7. Ji Chu,
  8. Xianchuang Pan,
  9. Ling Hu,
  10. Jian Li,
  11. Yuan Xu,
  12. Youpeng Zhong,
  13. Song Liu,
  14. Fei Yan,
  15. Dian Tan,
  16. R. Bachelard,
  17. C. J. Villas-Boas,
  18. Alan C. Santos,
  19. and Dapeng Yu
Quantum batteries are miniature energy storage devices and play a very important role in quantum thermodynamics. In recent years, quantum batteries have been extensively studied, but
limited in theoretical level. Here we report the experimental realization of a quantum battery based on superconducting qubits. Our model explores dark and bright states to achieve stable and powerful charging processes, respectively. Our scheme makes use of the quantum adiabatic brachistochrone, which allows us to speed up the {battery ergotropy injection. Due to the inherent interaction of the system with its surrounding, the battery exhibits a self-discharge, which is shown to be described by a supercapacitor-like self-discharging mechanism. Our results paves the way for proposals of new superconducting circuits able to store extractable work for further usage.

Suppressing Coherent Two-Qubit Errors via Dynamical Decoupling

  1. Jiawei Qiu,
  2. Yuxuan Zhou,
  3. Chang-Kang Hu,
  4. Jiahao Yuan,
  5. Libo Zhang,
  6. Ji Chu,
  7. Wenhui Huang,
  8. Weiyang Liu,
  9. Kai Luo,
  10. Zhongchu Ni,
  11. Xianchuang Pan,
  12. Zhixuan Yang,
  13. Yimeng Zhang,
  14. Yuanzhen Chen,
  15. Xiu-Hao Deng,
  16. Ling Hu,
  17. Jian Li,
  18. Jingjing Niu,
  19. Yuan Xu,
  20. Tongxing Yan,
  21. Youpeng Zhong,
  22. Song Liu,
  23. Fei Yan,
  24. and Dapeng Yu
Scalable quantum information processing requires the ability to tune multi-qubit interactions. This makes the precise manipulation of quantum states particularly difficult for multi-qubit
interactions because tunability unavoidably introduces sensitivity to fluctuations in the tuned parameters, leading to erroneous multi-qubit gate operations. The performance of quantum algorithms may be severely compromised by coherent multi-qubit errors. It is therefore imperative to understand how these fluctuations affect multi-qubit interactions and, more importantly, to mitigate their influence. In this study, we demonstrate how to implement dynamical-decoupling techniques to suppress the two-qubit analogue of the dephasing on a superconducting quantum device featuring a compact tunable coupler, a trending technology that enables the fast manipulation of qubit–qubit interactions. The pure-dephasing time shows an up to ~14 times enhancement on average when using robust sequences. The results are in good agreement with the noise generated from room-temperature circuits. Our study further reveals the decohering processes associated with tunable couplers and establishes a framework to develop gates and sequences robust against two-qubit errors.

Deterministic multi-qubit entanglement in a quantum network

  1. Youpeng Zhong,
  2. Hung-Shen Chang,
  3. Audrey Bienfait,
  4. Étienne Dumur,
  5. Ming-Han Chou,
  6. Christopher R. Conner,
  7. Joel Grebel,
  8. Rhys G. Povey,
  9. Haoxiong Yan,
  10. David I. Schuster,
  11. and Andrew N. Cleland
Quantum entanglement is a key resource for quantum computation and quantum communication cite{Nielsen2010}. Scaling to large quantum communication or computation networks further requires
the deterministic generation of multi-qubit entanglement \cite{Gottesman1999,Duan2001,Jiang2007}. The deterministic entanglement of two remote qubits has recently been demonstrated with microwave photons \cite{Kurpiers2018,Axline2018,Campagne2018,Leung2019,Zhong2019}, optical photons \cite{Humphreys2018} and surface acoustic wave phonons \cite{Bienfait2019}. However, the deterministic generation and transmission of multi-qubit entanglement has not been demonstrated, primarily due to limited state transfer fidelities. Here, we report a quantum network comprising two separate superconducting quantum nodes connected by a 1 meter-long superconducting coaxial cable, where each node includes three interconnected qubits. By directly connecting the coaxial cable to one qubit in each node, we can transfer quantum states between the nodes with a process fidelity of 0.911±0.008. Using the high-fidelity communication link, we can prepare a three-qubit Greenberger-Horne-Zeilinger (GHZ) state \cite{Greenberger1990,Neeley2010,Dicarlo2010} in one node and deterministically transfer this state to the other node, with a transferred state fidelity of 0.656±0.014. We further use this system to deterministically generate a two-node, six-qubit GHZ state, globally distributed within the network, with a state fidelity of 0.722±0.021. The GHZ state fidelities are clearly above the threshold of 1/2 for genuine multipartite entanglement \cite{Guhne2010}, and show that this architecture can be used to coherently link together multiple superconducting quantum processors, providing a modular approach for building large-scale quantum computers \cite{Monroe2014,Chou2018}.

A fast and large bandwidth superconducting variable coupler

  1. Hung-Shen Chang,
  2. Kevin J. Satzinger,
  3. Youpeng Zhong,
  4. Audrey Bienfait,
  5. Ming-Han Chou,
  6. Christopher R. Conner,
  7. Étienne Dumur,
  8. Joel Grebel,
  9. Gregory A. Peairs,
  10. Rhys G. Povey,
  11. and Andrew N. Cleland
Variable microwave-frequency couplers are highly useful components in classical communication systems, and likely will play an important role in quantum communication applications.
Conventional semiconductor-based microwave couplers have been used with superconducting quantum circuits, enabling for example the in situ measurements of multiple devices via a common readout chain. However, the semiconducting elements are lossy, and furthermore dissipate energy when switched, making them unsuitable for cryogenic applications requiring rapid, repeated switching. Superconducting Josephson junction-based couplers can be designed for dissipation-free operation with fast switching and are easily integrated with superconducting quantum circuits. These enable on-chip, quantum-coherent routing of microwave photons, providing an appealing alternative to semiconductor switches. Here, we present and characterize a chip-based broadband microwave variable coupler, tunable over 4-8 GHz with over 1.5 GHz instantaneous bandwidth, based on the superconducting quantum interference device (SQUID) with two parallel Josephson junctions. The coupler is dissipation-free, features large on-off ratios in excess of 40 dB, and the coupling can be changed in about 10 ns. The simple design presented here can be readily integrated with superconducting qubit circuits, and can be easily generalized to realize a four- or more port device.